1. Field of the Invention
The present invention is directed to a computer tomography apparatus, and in particular to such an apparatus having an annular anode surrounding a measuring field in which a patient is disposed, and means for causing an electron beam to orbit the patient while incident on the anode, so that x-radiation is generated in the measuring field from different directions.
2. Description of the Prior Art
Various types of computer tomography devices are known wherein a measuring installation consisting of an x-ray source and a radiation detector is mechanically moved around a measuring field in which the patient is disposed. Computer tomography devices of this type thus require the relatively heavy components to be mechanically moved for scanning the entire measuring field. Another type of computer tomography apparatus employs a ring-shaped anode which surrounds the measuring field, and a coil is provided through which the electron beam passes so as to deflect the electron beam onto a surface of the anode in an orbit around the patient. Trans-irradiation of the patient disposed in the measuring field from a number of different directions can thus be undertaken very quickly, so that a cross-sectional image of the slice of the patient disposed in the measuring field can be generated in an extremely short time.
The electron beam required for generating the x-radiation is emitted by an electron gun, and is electron-optically focused onto the anode surface by means of electrical and/or magnetic lenses. The required orbital movement of the electron beam is achieved using an electrical or magnetic deflecting field which is continually changed in orientation to deflect the electron beam as desired.
A focus of the electron beam on the anode surface which is as close to a point as possible is required for achieving a high image sharpness in the resulting x-ray image. A true point focus, however, would lead to local over heating at the anode surface, and would result in the destruction thereof. The focal spot must therefore be "stretched." The enlarged focal spot is ideally a rectangle having, for example, a side ratio of 6:1. If the surface of the anode on which the electron beam is incident is disposed at a small angle of, for example, 10.degree. relative to the emission direction of the x-radiation, the projection of the focus on this surface will have the required punctiform shape. The actual electron focal spot on the anode surface will thus be relatively large, and result in a correspondingly reduced thermal load on the anode, but will have optical properties corresponding to a relatively small focal spot.
In conventional orbiting type computer tomography devices, expanding the focal spot along its longitudinal direction requires relatively complex electron-optical auxiliary means.
A computer tomography apparatus is disclosed in German No. OS 28 11 464, corresponding to U.S. Pat. Nos. 4,130,759 and 4,158,142 wherein deflection of the electron beam in the radial direction on the annular anode is undertaken so that the electron beam is alternatingly incident on the anode and on a separate beam collector. The beam collector does not function to generate x-radiation. The path of the electron beam on the anode surface, accordingly, is not continuous, but is an interrupted path.